1. Field of the Invention
This invention relates to a secondary air pump control system for internal combustion engines, which controls the operation of an air pump which supplies secondary air into the exhaust passage of the engine to purify exhaust gases emitted from the engine, and more particularly to a secondary air pump control system of this kind, which controls the flow rate of secondary air supplied by the air pump and/or detects abnormality of the air pump, by calculating the flow rate of secondary air, based on values of voltage and current of electric power supplied to the air pump.
2. Prior Art
There is conventionally employed in internal combustion engines, a secondary air pump control system which controls the supply of secondary air into the exhaust passage of the engine by driving and controlling an air pump to supply a suitable amount of secondary air to oxidize noxious components, such as carbon monoxide, in exhaust gases emitted from the engine or activate a catalytic converter provided in the exhaust system, to thereby purify noxious components in exhaust gases.
The conventional secondary air pump control systems include one proposed by Japanese Utility Model Publication (Kokoku) No. 4-22025, which regulates the flow rate of secondary air by means of a secondary air supply system having a reed valve (check valve) and a mechanical pump.
However, the conventional secondary air pump control systems including the proposed one are not provided with flow rate-detecting means for detecting the flow rate of secondary air. Therefore, the proposed secondary air pump control system is incapable of controlling the flow rate of secondary air supplied into the exhaust passage to suitable values appropriate to operating conditions of the engine.
Since the proposed air pump control system employs a reed valve and a mechanical pump as mentioned above, accurate control of the flow rate of secondary air depending on operating conditions of the engine is difficult to achieve.
Further, if an air flow sensor is employed to detect the flow rate of secondary air to carry out the air pump control, the flow rate of secondary air can be accurately detected, which, however, results in a high manufacturing cost.
Furthermore, the arrangement of such an air flow sensor in the exhaust passage may cause a pressure loss, depending on the layout thereof, which may incur a decrease in the flow rate of secondary air.
If an 02 sensor is employed, in place of the air flow sensor, to detect the flow rate of secondary air to carry out the air pump control, the 02 sensor can do nothing more than detecting whether or not there is a supply of secondary air into the exhaust passage. Therefore, the 02 sensor is not suitable for use as the flow rate sensor for secondary air control which has to be carried out with high accuracy.
Besides, the conventional secondary air pump control systems including the proposed one do not contemplate providing a failure-diagnosing function. Therefore, a secondary air pump control system having a failure-diagnosing function has been demanded.